McGonigal says Web site owners — including those of major corporations — should make tasks challenging, so that the sense of achievement upon completing them is greater.

The assertion is related to McGonigal’s definition of a game, which she talked about at the Gamification Summit today in San Francisco. She said that “games are unnecessary obstacles that we volunteer to tackle.” When people play games, they are trying to achieve a feeling of “eustress,” a kind of positive stress that motivates us to perform our best, she said.

McGonigal, who has spawned some of the most creative games in the history of the industry (such as World Without Oil, a game aimed at getting people to reduce their use of oil), believes wholeheartedly in gamification’s ability to make big changes in the behavior of people who in turn can create big changes in the real world. But she worries that gamification efforts fall short when they adopt the mechanics of a game, such as giving rewards for certain behavior, without adopting the spirit of a game.

The good feeling you get when you play an engaging game is what she calls “gamefulness.”

“Lots of things have the bells and whistles, but not the heart of a game,” she said.

McGonigal actually created a game to go with her speech, to make it more fun. She had people in the audience send text messages to a phone number and do what it suggested. Ultimately, the game encouraged audience members to yell “amen” at various points during her speech.

She said that a lot of games have blurred the line between work and play. FarmVille, for instance, requires you to collect a lot of things in order to progress, much as you do for work. The common notion is that some jobs are hard work, and some of the best games are hard to play. The ultimate goal is to get gamers to conquer the game and get to a kind of euphoric feeling at the end of it.

Golf fits the definition of McGonigal’s idea of a game, since it creates unnecessary obstacles to putting a ball in a hole. It makes sense. If a game is too easy, it becomes boring.

“In real life, we would just drop a ball in a hole,” she said. “Why does it become a game when we make it harder?”

It has to do with creating that feeling of eustress, which drives our motivation and performance and keeps us from getting bored or uninspired. She notes that users have invested more than 5.93 million years of time into playing World of WarCraft, the popular online role-playing game. Getting to the highest level in WoW takes 600 hours.

While McGonigal was writing her book, she suffered a bad head injury. She was getting depressed because she couldn’t do things like read a book or write. Doctors were telling her that the head injury itself was making her biochemically prone to feelings of depression and she needed to pull out of it with positive feelings in order to heal.

So she created a game. Called Super Better, the game was aimed at helping patients recover from an illness. She credits it with saving her own life. McGonigal has created a company, Social Chocolate, to commercialize that game and to launch others that lead to real world benefits. She describes the effort at www.gameful.org.

Games that are done well create a feeling of urgent optimism, which is also the opposite of depression. It makes us feel like we are part of a social fabric, that we’re blissfully productive, and that we have achieved something bigger than ourselves with epic meaning, she said.